When checking whether a path is a valid repository path, we try to read
the "commondir" link file. In the process, we neither confirm that
constructing the file's path succeeded nor do we verify that reading the
file succeeded, which might cause us to verify repositories on an empty
or bogus path later on.
Fix this by checking return values. As the function to verify repos
doesn't currently support returning errors, this commit also refactors
the function to return an error code, passing validity of the repo via
an out parameter instead, and adjusts all existing callers.
